Method for calibrating an electrophoretic dispaly panel

ABSTRACT

The invention relates to a method for calibrating an electrophoretic display panel ( 1 ) comprising a plurality of pixels ( 2 ) capable of representing at least two optical states by receiving driving signals ( 30 ), comprising the steps of displaying a first calibration image ( 22 ) containing said optical states in a first arrangement on said electrophoretic display panel and providing driving signals ( 30 ) to said pixels ( 2 ) corresponding to a required image ( 23 ) resulting in a second calibration image ( 24 ) containing said optical states in a second arrangement on said electrophoretic display panel ( 1 ). The second calibration image ( 24 ) is compared with said required image ( 23 ) to determine differences ( 26 ) between said second calibration image ( 24 ) and said required image ( 23 ) and said driving signals ( 30 ) are adjusted in accordance with said differences such that said second calibration image ( 23 ) and said required image ( 24 ) match. By this method the uniformity of the electrophoretic display ( 1 ) is improved.

The invention relates to a method for calibrating an electrophoreticdisplay panel comprising a plurality of pixels capable of representingat least two optical states by receiving driving signals.

U.S. Pat. No. 2002/0,196,526 discloses an electrophoretic device,wherein a driving voltage is applied over a first and a second electrodeto allow electrophoretic particles to localize at either the first orthe second electrode by electrophoresis.

In more recent electrophoretic displays multiple optical states areobtained via e.g. time-weighted drive periods or division of the pixelsinto surfaces with different areas.

A problem associated with the known electrophoretic display panels isnon-uniformity, which is especially observed when changing from oneoptical state to another. In particular, the display panels have beenobserved to suffer from a form of image retention, whereby the actualgrey level of a pixel in a new image may depend upon the grey level ofthat pixel in a previous image. In such cases, a previous image may bepartially visible in a new image. These problems are believed to becaused by strong memory effects (bi-stability) and dwell time effects.The dwell time of a particular display pixels is generally defined asthe period in which no voltage was applied to that pixel.

It is an object of the invention to provide a method for calibrating anelectrophoretic display panel and in particular to reduce the form ofimage retention described above.

This object is achieved by the method comprising the steps of:

-   displaying a first calibration image containing said optical states    in a first arrangement on said electrophoretic display panel;-   providing driving signals to said pixels corresponding to a required    image resulting in a second calibration image containing said    optical states in a second arrangement on said electrophoretic    display panel;-   comparing said second calibration image with said required image to    determine differences between said second calibration image and said    required image;-   adjusting said driving signals in accordance with said differences    such that said second calibration image and said required image    match.

By providing driving signals corresponding to a required image butresulting in a second calibration image and comparing the secondcalibration image with the required image, pixels or groups of pixelscan be determined on the electrophoretic display for which thetransition of the optical states do not result in the required opticalstate of the required image. After having determined the differences,also referred to as artefacts, these artefacts are repaired by adjustingthe driving signals for the pixels in accordance with the differencesobserved, such that the required image is obtained. As a result thedisplay panel uniformity is improved and, more specifically, the effectsof image retention are reduced. This calibration method preferablyconstitutes a step in the manufacturing of an electrophoretic display.Preferably, the optical states are grey levels.

In a preferred embodiment of the invention the driving signalscorresponding to said required image are provided such that all possibleoptical state transitions are involved in comparison with said firstcalibration image. In this situation it is possible to determine allartefacts at once.

In a preferred embodiment of the invention said first arrangement andsaid second arrangement comprise one or more blocks of individual pixelsor groups of pixels of said display panel. The blocks may substantiallyentirely cover said electrophoretic display panel. By having severalrepeats of the calibration patterns of blocks distributed over theelectrophoretic display panel, lateral, i.e. variations across thedisplay panel, artefacts can be determined. As a result, the displaypanel uniformity is also improved. It may occur that the driving signalsneed to be adjusted differently for different locations on the displaypanel.

In a preferred embodiment the second calibration image is recorded by aCCD-camera to determine said differences between said second calibrationimage and said required image. The CCD-camera may record the secondcalibration image and therefore determine the deviations from therequired image for the entire display panel at once.

In a preferred embodiment of the invention the electrophoretic displaypanel comprises a look-up table with driving signals corresponding totransitions between said optical states for said pixels and said methodfurther comprises the step of modifying said look-up table in accordancewith said adjusted driving signals. Before calibration a default look-uptable may be present used in driving the pixels. It may appear that thisdefault look-up table needs to be modified after the determination ofthe artefacts for adjusting the driving signals. Preferably thesedriving signals relate to driving voltages, reset voltages and/orpre-pulse voltages and said adjustment involves modifying the magnitudeand/or duration of said voltages and/or changing or introducing periodsbetween the driving voltages and/or adding additional voltage pulses.This modification allows restoration of the optical states or greylevels in accordance with the required image.

In an embodiment of the invention the step of displaying said firstcalibration image comprises the steps of:

-   recording said first calibration image and comparing said first    calibration image with a further calibration image;-   adjusting said driving signals such that said first calibration    image and said further calibration image match.

By also recording the first calibration image, e.g. by the CCD-camera,information on the initial or first arrangement can be obtained. Thisfirst arrangement of the first calibration image may need to be adjustedin order to arrive at a suitable block of grey scale levels to obtain atransition for all possible optical states or grey levels on providingthe driving signals corresponding to the required image, as describedabove.

In an embodiment of the invention the method further comprises the stepof providing further driving signals to said pixels corresponding tofurther required images and resulting in further calibration images andcomparing at least one of said further calibration images with saidfurther required images. This may prove to be advantageous in improvingthe uniformity across the electrophoretic display panel. Moreover suchfurther calibration images may be used in situations wherein theprevious history of the pixels is important, i.e. not the previousimage, but two or more images ago. In this case a third calibrationimage may be used wherein each block is split into smaller blocks withdifferent optical states or grey levels.

In an embodiment of the invention the above method is repeated one ormore times after adjusting said driving signals. By such a repetition ofthe method it may be verified whether the adjusted driving signalsactually improved the uniformity of the electrophoretic display panel.

The invention also relates to a display device having an electrophoreticdisplay panel comprising a plurality of pixels capable of representingat least two optical states, said device comprising:

-   means for displaying a first calibration image containing said    optical states in a first arrangement on said electrophoretic    display panel;-   means to provide driving signals to said pixels corresponding to a    required image having as a result a second calibration image    containing said optical states in a second arrangement, and-   means for adjusting said driving signals to match said second    calibration image and said required image.

It should be noted that, although the above method and display deviceaccording to the invention involve a plurality of pixels, the inventionand aspects thereof as described above applies mutatis mutandis to amethod and display device for a single pixel as well.

In particular, the invention also relates to a method for calibrating anelectrophoretic display panel comprising a pixel capable of representingat least two optical states by receiving driving signals, comprising thesteps of:

-   displaying a first optical state for said pixel on said    electrophoretic display panel;-   providing a driving signal to said pixel corresponding to a required    optical state having as a result said first optical state or a    second optical state for said pixel on said electrophoretic display    panel-   comparing said resulting first or second optical state with said    required optical state for said pixel to determine a difference    between said resulting first or second optical state and said    required optical state;-   adjusting said driving signal in accordance with said difference    such that said resulting first or second optical state and said    required optical state of said pixel match.

Moreover, the invention relates to a display device having anelectrophoretic display panel comprising a pixel capable of representingat least two optical states by receiving driving signals, said devicecomprising:

-   means for displaying a first optical state for said pixel on said    electrophoretic display panel;-   means to provide a driving signal to said pixel corresponding to a    required optical state having as a result said first optical state    or a second optical state for said pixel on said electrophoretic    display panel, and-   means for adjusting said driving signal to match said resulting    first optical state or second optical state and said required    optical state.

Accordingly, a single pixel display device can be calibrated. Of course,the driving signal may have as a result that the second optical statealready matches the required optical state, in which case the secondoptical state is the required optical state. Further, preferably theoptical states are grey levels.

U.S. Pat. No. 6,473,065 discloses methods for improving displayuniformity of organic light emitting displays by calibrating individualpixels. In this publication only lateral non-uniformity variations areadjusted for, whereas according to the invention primarily optical statetransitions are adjusted. Moreover, as the prior art methods are aimedat organic displays instead of electrophoretic displays, a singlemeasurement is sufficient for improving the uniformity, since nosubstantial memory effects for uniformity occur for organic pixels.

In contrast, for electrophoretic displays strong memory effects ariseresulting in the need for generating at least two calibration images.

The invention will be further illustrated with reference to the attacheddrawings, which show preferred embodiments of the invention. It will beunderstood that the device and method according to the invention are notin any way restricted to this specific and preferred embodiment.

In the drawings:

FIG. 1 shows a schematic illustration of an electrophoretic displaypanel;

FIG. 2 shows a cross-section view along II-II in FIG. 1;

FIG. 3 shows a schematic illustration of a set-up for performing themethod according to an embodiment of the invention;

FIG. 4 shows examples of calibration images and a required imageaccording to an embodiment of the invention;

FIG. 5 shows examples of adjusted driving signals as a result of themethod according to an embodiment of the invention, and

FIG. 6 shows a schematic illustration of a display device with anelectrophoretic display panel comprising a single pixel.

FIGS. 1 and 2 show an embodiment of an electrophoretic display panel 1of a device D having a first substrate 8, a second opposed substrate 9and a plurality of pixels 2.

Preferably, the pixels 2 are arranged along substantially straight linesin a two-dimensional structure. Other alternatives include e.g. ahoneycomb structure. An electrophoretic medium 5, having chargedparticles 6, is present between the substrates 8 and 9. In FIG. 2 thefirst substrate 8 has for each pixel 2 a first electrode 3, and thesecond substrate 9 has for each pixel 2 a second electrode 4. Theelectrodes 3, 4 are adapted to receive a driving signal from drive means10. The charged particles 6 are able to occupy extreme positions nearthe electrodes 3,4 and intermediate positions in between the electrodes3,4. In this way different optical states can be obtained. Hereinafter,these optical states are assumed to be grey levels. Each pixel 2 has anappearance determined by the position of the charged particles 6 betweenthe electrodes 3,4 for displaying the picture or image. Electrophoreticmedia 5 are known per se from e.g. U.S. Pat. No. 5,961,804, U.S. Pat.No. 6,120,839 and U.S. Pat. No. 6,130,774 and can e.g. be obtained fromE Ink Corporation. As an example, the electrophoretic medium 5 comprisesnegatively charged black particles 6 in a white fluid. When the chargedparticles 6 are in a first extreme position, i.e. near the firstelectrode 3, as a result of the potential difference being e.g. 15Volts, the appearance of the pixel 2 is e.g. white. Here it isconsidered that the picture element 2 is observed from the side of thesecond substrate 9. When the charged particles 6 are in a second extremeposition, i.e. near the second electrode 4, as a result of the potentialdifference being of opposite polarity, i.e. −15 Volts, the appearance ofthe pixel 2 is black. When the charged particles 6 are in one of theintermediate positions, i.e. in between the electrodes 3,4, the pixel 2has one of the intermediate appearances, e.g. light grey and dark grey,which are grey levels between white and black. The drive means 10 isarranged for driving each pixel 2 by supplying appropriate voltages tothe electrodes 3, 4 using a look-up table (LUT) 11. Appropriate drivingsignals are e.g. described in the non-prepublished patent application EP03100133 of the applicant. In an active matrix embodiment, the pixel mayfurther comprise switching electronics comprising for example thin filmtransistors (TFTs), diodes or MIM devices.

Further the display device D comprises means 12 for calibrating theelectrophoretic display panel I according to an embodiment of theinvention. The means 12 are arranged to communicate with the drive means12 to generate driving signals.

FIG. 3 shows a schematic illustration of a set-up 20 for performing themethod according to an embodiment of the invention. The set-up 20comprises the electrophoretic display panel 1 shown in FIGS. 1 and 2,drive means 10 and a CCD-camera 21.

The operation of the set-up 20 will be described with reference to FIG.4, showing a first calibration image 22, a required image 23 and asecond calibration image 24. The images 22, 23 and 24 are divided inarrangements of blocks 25 of pixels 2 covering the entire display panel1. Alternatively a multiplicity of such arrangements may be distributedover the electrophoretic display panel 1 to visualize lateralnon-uniformity effects.

The first arrangement for the first calibration image 22 is such that itcomprises all possible, in this case four, grey levels, indicated by thewhite (W), light grey (LG), dark grey (DG) and black (B) blocks 25. Thesecond arrangement of the second calibration image 24 is chosen suchthat in the transition from the first calibration image 22 to the secondcalibration image 24 all grey level transitions are involved. That is,for compliance to the required image 23, the upper four blocks 25 shouldall switch to W, the subsequent four blocks 25 to LG, the next fourblocks 25 to DG and the bottom four blocks 25 to B. Further calibrationimages may be displayed if need be. In this manner, memory effectspersisting over more than one image update may also be corrected for. Itis further noted that different arrangements for the calibration imagesare possible, depending upon details of the display I and the resolutionof the optical measurement system 21.

In operation the fabricated electrophoretic display is placed under anoptical imaging system, such as the CCD-camera 21. Then the display 1may be initialized to a well-defined state by providing particulardriving signals from the drive means 10. Next the first calibrationimage 22 is generated on the display panel 1 and the brightness of thegrey levels for the pixels 2 is recorded by the CCD-camera 21. If thebrightness of the initial grey levels is not correct the driving signalsare adjusted in accordance with the results of the measurements for theCCD-camera 21. The adjustments may be stored in the LUT 11. The displaypanel I may be initialized once more and the first calibration image 22may be re-displayed until the correct brightness levels are obtained asshown in FIG. 4.

Subsequently driving signals are provided corresponding to a requiredimage 23 resulting in the second calibration image 24. By comparing thesecond calibration image 24 with the required image 23, differences 26arising from image retention and other effects can be determined betweenthe second calibration image 24 and the required image 23. The requiredimage 23 represents the ideal image when all grey level transitions weresuccessfully obtained. Here the artefacts are twofold, the transition Bto W yielded a not entirely white block 25, whereas the transition W toDG yielded a too dark block 25.

The driving signals are adjusted in accordance with the differences 26for the pixels 2 of the blocks 25. This adjustment may be achieved bymodifying the LUT 11 of the drive means 10.

If necessary the display 1 may be re-initialized and the method may berepeated with new driving signals.

The display device D may comprise means 12, such as a button, to displaythe first calibration image 22. Subsequently, e.g. by pushing or turningthe button 12, driving signals are provided corresponding to therequired image 23 having as a result the second calibration image 24.Finally the means 12 or other means can be used to adjust the drivingsignals as to match the second calibration image 24 and the requiredimage 23. Accordingly, means are provided to enable consumers tocalibrate the electrophoretic display panel 1. It should be appreciatedthat the means 12 may comprise a plurality of control means forperforming the calibration steps described above.

FIG. 5 shows an example of suitable driving signals 30 for the pixels 2of the electrophoretic display panel 1. These driving signal includepre-pulse voltages 31, driving voltages 33 and optionally reset voltages32. The pre-pulse voltages 31 may release the particles 6 from theirextreme positions near the electrodes 3, 4 without enabling theparticles to substantially transfer to the other electrode 3, 4. Thereset voltages 32 may reduce the dependence of a pixel 2 on the previousappearance or representation because the particles 6 substantiallyoccupy an extreme position. It is noted that the time during which thereset voltage 32 is applied may be extended as described in thenon-pre-published patent application EP 03100133 of the applicant. Thedriving voltage 33 transfers the particles 6 to the positioncorresponding to the image information for the pixel 2. Adjustment ofthe driving signals 30 to calibrate the display panel I may includeadjusting the magnitude and duration of the pre-pulse voltages 31 and/orthe reset voltages 32 and/or the driving voltages 33, but may alsoinvolve changing or introducing periods between the driving voltages 33in the dwell time and/or introducing additional voltage pulses. Thisadjustment is preferably performed by modifying the LUT 11.

Finally, in FIG. 6 a display device D is shown comprising anelectrophoretic display panel 1 having a single pixel 2 capable ofrepresenting at least two optical states. The display device D comprisesmeans 12 to control the calibration of the display panel 1. Again, itshould be appreciated that the means 12 may comprise a plurality ofcontrol means for performing the calibration steps.

First the means 12, such as a button, are employed to display a firstoptical state for the single pixel 2 on the electrophoretic displaypanel 1. Subsequently, the button 12 is manipulated to provide a drivingsignal to the pixel 2 corresponding to a required optical state. Thedriving signal results in either the first optical state or a secondoptical state, which result is compared with the required optical state.If the resulting first or second optical state differs from the requiredoptical state, the button 12 may be employed to adjust the drivingsignal from the drive means 10 to match the second optical state and therequired optical state. Of course, the set-up 20 displayed in FIG. 2employing a CCD-camera 21 may be used as well.

1. A method for calibrating an electrophoretic display panel (1)comprising a plurality of pixels (2) capable of representing at leasttwo optical states by receiving driving signals (30), comprising thesteps of: displaying a first calibration image (22) containing saidoptical states in a first arrangement on said electrophoretic displaypanel; providing driving signals (30) to said pixels (2) correspondingto a required image (23) resulting in a second calibration image (24)containing said optical states in a second arrangement on saidelectrophoretic display panel (1); comparing said second calibrationimage (24) with said required image (23) to determine differences (26)between said second calibration image (24) and said required image (23);adjusting said driving signals (30) in accordance with said differencessuch that said second calibration image (23) and said required image(24) match.
 2. The method according to claim 1, wherein said opticalstates are grey levels.
 3. The method according to claim 1, wherein saiddriving signals corresponding to said required image are provided suchthat all possible optical state transitions are involved in comparisonwith said first calibration image.
 4. The method according to claim 1,wherein said first arrangement and said second arrangement comprise oneor more blocks (25) of individual pixels or groups of pixels of saiddisplay panel.
 5. The method according to claim 4, wherein said blockssubstantially entirely cover said electrophoretic display.
 6. The methodaccording to claim 1, further comprising the step of recording saidsecond calibration image by a CCD-camera (21) to determine saiddifferences between said second calibration image and said requiredimage
 7. The method according to claim 1, wherein said electrophoreticdisplay panel comprises a look-up table (11) with driving signalscorresponding to transitions between said optical states for said pixelsand said method further comprises the step of modifying said look-uptable in accordance with said adjusted driving signals.
 8. The methodaccording to claim 1, wherein said driving signals comprise drivingvoltages (33), and/or reset voltages (32) and/or pre-pulse voltages (31)and said adjustment involves modifying the magnitude and/or duration ofsaid voltages and/or changing or introducing periods between the drivingvoltages and/or introducing additional voltage pulses.
 9. The methodaccording to claim 1, wherein said step of displaying said firstcalibration image (22) comprises the steps of: recording said firstcalibration image and comparing said first calibration image with afurther calibration image; adjusting said driving signals such that saidfirst calibration image and said further calibration image match. 10.The method according to claim 1, wherein said method further comprisesthe step of providing further driving signals to said pixelscorresponding to further required images and resulting in furthercalibration images and comparing at least one of said furthercalibration images with said further required images.
 11. The methodaccording to claim 1, wherein said method is repeated one or more timesafter adjusting said driving signals.
 12. A display device (D) having anelectrophoretic display panel (1) comprising a plurality of pixels (2)capable of representing at least two optical states, said devicecomprising: means (12) for displaying a first calibration image (22)containing said optical states in a first arrangement on saidelectrophoretic display panel (1); means (12) to provide driving signals(30) to said pixels corresponding to a required image (23) having as aresult a second calibration image (24) containing said optical states ina second arrangement, and means (12) for adjusting said driving signals(30) to match said second calibration image (24) and said required image(23).
 13. The display device (D) according to claim 12, wherein saidoptical states are grey levels.
 14. The display device (D) according toclaim 12, wherein said device further comprises a look-up table (11)with driving signals corresponding to transitions between said opticalstates for said pixels and said means for adjusting said driving signalsare adapted to modify said look-up table in accordance with saidadjusted driving signals.
 15. A method for calibrating anelectrophoretic display panel (1) comprising a pixel (2) capable ofrepresenting at least two optical states by receiving driving signals(30), comprising the steps of: displaying a first optical state for saidpixel on said electrophoretic display panel (1); providing a drivingsignal (30) to said pixel corresponding to a required optical statehaving as a result said first optical state or a second optical statefor said pixel on said electrophoretic display panel comparing saidresulting first or second optical state with said required optical statefor said pixel to determine a difference between said resulting first orsecond optical state and said required optical state; adjusting saiddriving signal in accordance with said difference such that saidresulting first or second optical state and said required optical stateof said pixel match.
 16. A display device (D) having an electrophoreticdisplay panel (1) comprising a pixel (2) capable of representing atleast two optical states by receiving driving signals (30), said devicecomprising: means (12) for displaying a first optical state for saidpixel on said electrophoretic display panel (1); means (12) to provide adriving signal (30) to said pixel corresponding to a required opticalstate having as a result said first optical state or a second opticalstate for said pixel on said electrophoretic display panel, and means(12) for adjusting said driving signal (30) to match said resultingfirst optical state or second optical state and said required opticalstate.